Thermoplastic polymers, that is polymers which have a Tg above room temperature, and more preferably polymers which melt above about 100.degree. C., have been known for a considerable length of time. Unfortunately many thermoplastic polymers tend to fail in a brittle mode; that is the polymer tends to fail due to crack propagation and there is not significant polymer deformation.
There are a number of approachs to reduce the brittleness of thermoplastics by either copolymerizing or grafting the thermoplastic polymer onto a rubbery polymer, such as high impact polystyrene (HIPS) or by blending an impact modifier in the thermoplastic polymer.
Polymer morphology is extremely important if the reduction in brittleness is to be achieved. For example in U.S. Pat. Nos. 4,172,859 and 4,174,358 polyesters or polycarbonates may be toughened by physically blending into the polyester or polycarbonate a modifier polymer which adheres (reacts) with the thermoplastic and which is distributed so that it has a particle size from 0.01 to 1.0 micron.
The manufacture of HIPS requires dissolving a rubbery polymer in a monomer which is then polymerized to form a thermoplastic polymer. Epstein teaches dispersing (not forming a miscible blend of a rubbery polymer in a thermoplastic matrix using an extruder. In both cases it is essential to control the shear exerted on the mixture of thermoplastic and rubbery polymer to obtain the required particle size distribution and morphology. According to the present invention, while shear may be commercially used to provide good mixing of the miscible components the phase separation depends on, and is caused by altering the temperature conditions.
There is another method for creating/controlling the particle size distribution within a matrix. A blend of miscible polymers may undergo thermally induced phase separation as discussed in "Thermally Induced Phase Separation Behaviour of Compatible Polymer Mixtures" T. Nishi, T. T. Wang, and T. K. Kwei Macromolecules Vol. 8 No. 2 March-April 1975 page 227. These studies have focused on the optical properties of the polymer blends. To the best of applicants knowledge this procedure has not been considered for modifying the brittleness of thermoplastic polymers. Surprisingly, applicants have discovered that the thermally induced phase separation of a blend of a thermoplastic and a rubbery polymer results in a polyblend having reduced brittleness. It is not necessary that the rubbery phase be grafted to or chemically combined with the thermoplastic polymer.